Variables influencing development of single dose (24 hour) tolerance to morphine analgesia will be examined in rats. These variables will include the presence or absence of painful stimulation following the initial morphine injection, similarity of postinjection environment to the subsequent testing environment, and temporal distribution of postinjection pain stimuli. A second approach will be to examine changes in multiple unit activity recorded from periaqueductal gray and caudate nucleus following a series of twice daily morphine injections. Sites from which analgesia can be produced by electrical stimulation through the recording electrodes will be compared to neutral sites. In some animals morphine will be microinjected into the recording site to determine direct effects of morphine on multiple unit activity at sites from which analgesia can be produced by microinjection of morphine to neutral sites.